The driver of a San Francisco airport shuttle bus -- O.J. Simpson's brother -- may have fallen asleep at the wheel before the vehicle crashed Friday on Interstate 280, killing a passenger, authorities said Monday.

Melvin Leon Simpson, 58, of San Mateo, was at the end of his shift and was suffering flu-like symptoms when his 21-seat SFO Airporter shuttle bus struck a guard rail and veered into a concrete support column near Mariposa Street, killing 57-year-old Lynne de Matties of Phoenix, authorities said.

De Matties, who was arriving in San Francisco for a Valentine's weekend vacation, was thrown from the van and died instantly. Six people were injured, including de Matties' husband and a tourist from New York who lost two fingers in the crash.

None of the passengers was wearing a seat belt, as the van was not required to have any, officials said.

Simpson first told investigators that the dead woman was a homeless person who had wandered onto the freeway. But later, when he realized a passenger was a victim, Simpson said he had swerved to avoid a car that cut in front of him.

Passengers interviewed by California Highway Patrol investigators say they never saw a woman or a car that swerved in front of the bus. "They just said they felt the van move to the right, then drift off to the left," said CHP Officer Shawn Chase.

"No one says they saw a car, so the likelihood of him dozing off is probably pretty good," Chase said. He added, however, that no passenger had seen Simpson asleep. "You can't say for sure," he said.

Chase said Simpson had not been feeling well Friday and had considered not coming into work. He had been driving since 3 a.m. that day, and the accident happened at 11 a.m.

Simpson provided a sample of his blood to investigators to determine whether alcohol or drugs were a factor in the crash, Chase said.

"We asked him for a voluntary sample, and he gave it," Chase said. "We don't suspect there to be anything in it."

Simpson is the older brother of O.J. Simpson, the onetime star football running back who grew up in San Francisco and graduated from Galileo High School. Years after his pro career ended, O.J. Simpson was tried for murder in the deaths of his wife and her friend, but was acquitted.

Melvin Simpson has worked for SFO Airporter for more than five years and has a clean driving record, the CHP and company officials said.

Reached by phone Monday at his San Mateo home, Melvin Simpson said he could not make a detailed statement about the crash.

"I'm sorry," he said. "Nobody could be more sorry.

"The accident was awful," he said. "I'm still in shock."

De Matties, 57, was killed instantly when she hit her head against the support column and then was ejected from the shuttle.

Nanci Pratt, Nick de Matties' ex-wife, said Lynne de Matties was a vice president of a company in Scottsdale and had two children by a previous marriage.

"She was a very lovely woman -- it's just horrible," Pratt said.

Susan Saidenberg, 56, of New York lost two fingers in the accident and remained in fair condition Monday. The other passengers, who were treated for minor to moderate injuries and released from the hospital, were Chris Losee, 50, and Sherm Losee, 52, of Ogden, Utah, and Anne Kelly, 51, of Edmond, Okla.

Melvin Simpson also suffered minor injuries.

Jeff Leonoudakis, vice president of SFO Airporter in South San Francisco, said his company is doing its own investigation. Melvin Simpson has been removed from his driving job pending the outcome of the investigation.

He said Melvin Simpson was a good employee who had had no problems with the company.

"It's incredibly sad. What can you say?" Leonoudakis said. "Your thoughts and prayers go out to everyone who was involved in this thing."

The company was started in 1976. "We have an excellent safety record," Leonoudakis said. "This is the first time we have ever had an accident of this nature, remotely close to it."